ABSTRACT
Multi-copy gene systems that evolve within, as well as between, individuals are common. They include viruses, mitochondrial DNAs, multi-gene families etc. The paradox is that neutral evolution in two stages should be far slower than single-copy systems but the opposite is often true, thus leading to the suggestion of natural selection. We now apply the new Generalized Haldane (GH) model to quantify genetic drift in the mammalian ribosomal RNA genes (or rDNAs). On average, rDNAs have C ∼ 150 - 300 copies. A neutral mutation in rDNA should take 4NC* generations to become fixed (N, the population size; C*, the effective copy number). While C > C* >> 1 is expected, the observed fixation time in mouse and human is < 4N, hence the paradox of C*< 1. Genetic drift thus appears as much as 100 times stronger for rRNA genes as for single-copy genes. The large increases in genetic drift are driven by a host of molecular mechanisms such as gene conversion and unequal crossover. Although each mechanism of drift has been extremely difficult to quantify, the GH model permits the estimation of their total effects on genetic drift. In conclusion, the GH model can be generally applicable to multi-copy gene systems without being burdened by tracking the diverse molecular mechanisms individually.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
This version of the manuscript has been revised to include updates to the author list and substantial modifications to the Introduction and Discussion sections. The revisions clarify that our approach aims to remove the total effects of random genetic drift, rather than the details of each molecular mechanism. to focus on more biological interesting evolutionary forces such as selection and mutation.
